The present invention relates generally to apparatus for detecting whether a sheet of paper has been picked up by the finger members of a printing press or the like and particularly to vacuum flow sensors.
In a printing press or the like, it is desired to move a sheet of paper from one position to another such as from an unprinted stock of paper to the printing press or from the printing press to a printed stock location. This is typically accomplished by utilizing finger members connected to a vacuum source through a vacuum passageway and which pick up sheets of paper by vacuum. To allow mechanized operation of the printing press, it is necessary to detect whether or not the finger members have picked up a sheet of paper and to provide suitable input to the printing press controls. Initially this was accomplished by pure mechanical controls which included structure which deflected according to the amount of vacuum present in the line and by mechanical linkage from the structure controling the press operation. This was later replaced by devices having membranes which measured the amount of vacuum present in the line and closed a switch providing an electrical signal to the press controls. However, such prior vacuum measuring devices were disadvantageous for several reasons. Specifically, such devices had a finite life, were subject to dust accumulation, and were subject to the generation of false signals due to change in the paper stock, changes in the vacuum source, and the like.
Thus, a need has arisen for providing accurate and sensitive input to the press controls of whether or not a sheet of paper has been picked up by the finger members.